Magellan Begins Termination Activities

From the "JPL Universe"
September 9, 1994

Magellan begins termination activities

JPL's Magellan spacecraft on Sept. 6 began a "windmill"
experiment, testing the amount of pressure needed to keep it from
spinning in Venus's upper atmosphere. The experiment begins the
final termination activities for the spacecraft, expected to burn
up by Oct. 14.

Magellan was launched in May 1989 and began mapping the surface
of the planet in September 1990. In three cycles, each comprised
of one Venus day or 243 Earth days, it mapped with imaging radar
98 percent of the planet's surface.

In three subsequent cycles, it has observed Venus' gravity over
95 percent of the planet, gathering data so scientists can map the
planet's interior. This period also included the "aerobraking" of
the spacecraft into a near-circular orbit for final gravity
operations in the last two cycles.

In recent weeks, the spacecraft's solar arrays have experienced
losses due the extremes of temperature as the spacecraft went from
direct sunlight to shadow in its orbit. The thermal stress after
more than five years in space has caused degeneration of solar
cells and has brought the spacecraft near the end of its useful
life, said Project Manager Doug Griffith.

Controllers sent commands in late August to begin orbit trim
maneuvers to reduce the altitude of the spacecraft from a near-
circular orbit of 197 km by 541 km (123 by 338 miles) to an orbit
of 172 km by 390 km (107 by 242 miles). These altitude reductions
were required to setup conditions for the final experiment phases.

For the "windmill" experiment, the spacecraft's wing-like solar
arrays are turned in opposite directions, like windmill sails, to
receive pressure of upper atmosphere molecules. The experiment is
testing to see how much torque will be needed to keep the
spacecraft from spinning on its axis, Griffith said. These data
will allow engineers and scientists to better understand gas-
surface interactions and gain additional aerodynamic and
atmospheric data for future mission design.

The windmill experiment is scheduled to last until Sept. 14.
Two weeks later, more orbit trim maneuvers are scheduled to lower
the spacecraft altitude to prepare for the final termination
experiment.

Three further trim maneuvers will change the altitude by 8 to 9
km (5 to 6 miles) each on Oct. 10, which will place the altitude
of periapsis at 155 km (96 miles). It will be lowered finally to
136 km (85 miles) on Oct. 12. Gravity data acquisition will
continue during all these periods up until Oct. 10.

"After Oct. 12, Magellan will permanently enter the atmosphere
in about two days, possibly in one day," Griffith said. The
atmosphere will drag the spacecraft toward the surface of the
planet, but it will burn up high in the skies over Venus, he
added.

Two things may happen, Griffith explained. The spacecraft will
overheat, or its control thrusters will be unable to maintain
pointing control. In either circumstance, project controllers will
lose communication with Magellan.

Using synthetic aperture radar, or side-looking radar, the
Magellan project has mapped almost all of the surface of Venus at
high resolution, providing maps of its unique pancake domes of
lava, strange volcanic structures, craters and high mountains.

Additionally, the project has provided a gravity map of the
planet and contributed to an ongoing study of its massive
atmosphere of carbon dioxide and high sulfuric acid clouds.

The data that streamed back from its radar image and gravity-
data acquisition during six cycles has built a gigantic database
of new knowledge about Venus and the formation of the solar system
which will be studied by scientists for decades to come, project
officials said.

Magellan coming to the end of a long and productive life

Magellan will end its mission operations in mid-October, when the
spacecraft enters the atmosphere of Venus and burns up. The
mission has far exceeded all of its primary and extended-mission
objectives and has produced more image data than all previous U.S.
planetary missions combined.

By Phil Allin, Magellan Project

In recent months it appeared that Magellan might become the first
functioning planetary spacecraft to be abandoned in space. Due to
the failure of the on-board tape recorder "A" and the subcarrier
modulation of transmitter A, and the degraded performance of
transmitter B, Magellan is incapable of resuming radar mapping.

But Magellan is not "dead." It continues to maintain precise
attitude control and a strong carrier signal for communication
with Earth.

It has been returning global gravity data from circular orbit
since early August 1993 and will complete a high-resolution
gravity map covering 95 percent of Venus on Oct. 9. Then it will
be commanded to lower its orbit in a series of steps that will
cause it to enter the atmosphere. Thus it may become the first
planetary spacecraft to be terminated intentionally.

On other planetary missions the spacecraft usually quit
operating due to the failure of a critical system or the depletion
of some limited resource such as attitude-control propellant.

For example, when Mariner 2 went to Venus in 1962, several of
the critical components became hotter than their design limits.
When it went silent about a month after the successful flyby, it
was believed that the battery or earth sensor had failed. Many of
the later Mariners failed when they ran out of the nitrogen gas
used for attitude control.

When the Pioneer Venus Orbiter ran out of propellant in October
1992 after 14 years in space, controllers could not raise the
periapsis out of the Venus upper atmosphere and it burned up as it
entered.

Some planetary spacecraft have been designed for long,
quiescent cruise periods, maintaining a fixed attitude with
reference to the Earth and reference stars. In this way, Voyagers
1 and 2 can continue their search for the heliopause while
expending very little propellant. The demise of the Voyagers is
expected to come when the output from the radioisotope
thermoelectric generators (RTGs) is insufficient to power the
critical on-board electronics--in 20 to 30 years.

Magellan is a different kind of bird. Due to the almost
constant maneuvers required during the radar-mapping phase of the
mission, the spacecraft is controlled by reaction wheels. These
electrically driven momentum wheels allow for smooth movement in
each of the three axes. This system requires frequent
"desaturations" (adjustment of spin rate in the wheels) and
"starcals" (verification of the attitude by scanning reference
stars).

During brief periods in which the forces on the spacecraft
might exceed the control of the reaction wheels--such as the Venus
orbit insertion or the recent aerobraking passes--the spacecraft is
switched to thruster control. The thrusters are also used during
desaturations, but Magellan carries far too little propellant to
remain on thrusters for an extended period.

Perhaps the greatest risk to Magellan is the failure of one of
the remaining gyros. Gyro A2 is presently at 52,000 hours of
operating life. The gyros are designed to have an operating
lifetime between 40,000 and 60,000 hours. However, gyro B2 failed
with less than 20,000 hours. If either of the remaining gyros
fail, the spacecraft would be unable to maintain its attitude in
space and would tumble out of control.

Another threat is the degradation of the solar panels. Magellan
is in a period of solar occultation, passing through the shadow of
Venus on each orbit.

Fifteen times a day the panels are subjected to a 190 C
temperature cycle. As a result, the electrical connections between
the solar cells are breaking, reducing the power output. The
power has to be managed by periodically turning off some of the
spacecraft electronics.

Analysis of the present propellant reserves show that, even if
all the remaining propellant were used to raise the orbit, the
spacecraft will enter and burn up in the Venus atmosphere in about
five years.